Photosensitive flame monitoring circuit



May 23, 1967 D. c. K. G. INNES 3,321,634.

I PHOTOSENSITIVE FLAME MONITORING CIRCUIT Filed Aug. 17, 1964 2 Sheets-Sheet l FIG! INVENTOR. David C. K.G. Innes ATYT O-RN EY D. c. K. G. INNES PHOTOSENSITIVE FLAME MONITORING CIRCUIT Filed Aug. 17, 1964 Sheets-Sheet 2 v Hag- 670 United. States Patent v O 3,321,634 PHOTOSENSITIVE FLAME MONITORING CIRU1T David C. K. G. lnnes, London, England, assignor to Babcock 8: Wilcox, Limited, London, England, a corporation of Great Britain Filed Aug. 17, 1964, Ser. No. 390,100 8 Claims. (Cl. 250215) This invention relates to electronic circuits suitable for use in flame monitoring apparatus and to calculating networks having an application in such apparatus.

For monitoring the burners of water tube boiler furnaces, for example, photo conductive cells may be used which are placed at the outer ends of sight tubes in line with the flame fronts of the burners and responding ac cording to the light radiation from the flame to actuate desired indicating or/and alarm or/and control means, as for example, means to sound an alarm and to cut off fuel supply to a burner if combustion at that burner becomes inefficient. Since an eflicient or healthy flame tends to produce a characteristic flicker or high frequency fluctuation, the flicker response of the cell is useful for monitoring, but preferably the flicker component of the cell illumination should be compared with the average illumination of the cell to compensate for reduced cell flicker responses which are not due to actual reduction in flame flicker but are rather due merely to possible deposits on sight glasses of the tube. The difliculty arises, however, that at any rate with many photo conductive cells, the amplitude of the cell flicker response becomes disproportionately less at lower average cell illuminations owing to a less quick cell response to the individual flickers of the received radiation. An object of the present invention is to provide an electronic circuit which, in spite of such phenomena, will produce an output representative or substantially so of the ratio of flame flicker to average flame brightness.

The present invention includes a circuit for flame monitoring apparatus adapted to provide a signal representative of the ratio of flame fluctuations to average flame brightness including a photo-conductive cell, a silicon junction diode or device of equivalent character connected in series with the cell and arranged substantially to compensate by its increased A.C. resistance at lower currents for diminished flicker response of the cell at lower illuminations, and means for passing an A.C. signal representative of radiation fluctuations at the cell to a second silicon junction diode or device of equivalent character receiving a current controlled by the cell and representa tive of average illumination of the cell and providing an A.C. output signal.

, The invention also includes a circuit for flame monitoring apparatus adapted to provide a signal dependent upon flame radiation fluctuations at a photo-conductive cell and including a silicon junction diode or device of equivalent character connected in series with the cell and arranged substantially to compensate by its increased A.C. resistance at lower currents for diminished flicker response of the cell at lower illuminations.

The invention moreover includes a circuit for flame monitoring apparatus adapted to provide a signal representative of the average flame radiation at a photo-conductive cell and including a silicon junction diode or device of equivalent character connected in series with the cell and arranged through its increased A.C. resistance at lower currents to increase the sensitivity of the said signal to changes in cell illumination at lower illuminations.

The invention also includes a calculating network adapted to take a ratio of an A.C. signal to a DC. signal comprising a silicon diode or device of equivalent character, for example, a gold-bonded germanium diode, fed with DC. from a high resistance source and with A.C. from a high impedance source.

The invention will now be described by Way of example with reference to the accompanying drawings, in which-- FIGURE 1 is a sectional side elevation of a pulverized coal burner arranged at a combustion chamber firing port and having an oil burner arranged co-axially thereof, and

FIGURE 2 shows an electronic circuit for apparatus for monitoring the flame of a pulverized coal and oil burner of the type generally shown in FIG. 1.

Referring to FIGURE 1 of the drawings, for monitoring the flames of a pulverized coal burner 1 arranged at a firing port in a water-cooled side wall 3 of a combustion chamber 4 and of an oil lighting-up and auxiliary burner 5 arranged on the axis thereof, monitoring apparatus is provided which includes an inclined sighting pipe 6 arranged in sighting relationship to the flame fronts 13 and 24 respectively produced by the pulverized coal burner 1 and the auxiliary burner 5.

The pulverized coal burner is of the kind which tends to give a flame surrounding a space or hollow core near the burner and about the axis thereof and in which pulverized coal entrained by a stream of primary combustion air is conducted via conduit 18 and through elbow 19 to the firing port in a pipe 7 which extends axially of the firing port. The pipe 7 extends through a windbox 8 on the outer side of the combustion chamber wall 3 and the pulverized coal and primary air, shortly after leaving the open end 9 of the pipe 7, impinge upon a pulverized fuel burner vaned diffuser 10 which is adapted to give radially outwardly directed components of motion to pulverized coal particles as well as to promote mixing between the primary air and secondary air that flows around the diffuser 10 and comes from the windbox 8. The secondary air is under control of an air register 11 comprising an annular row of rotatable vanes 12. When the pulverized coal burner 1 alone is operated and the coal flame therefrom is normal, ignition of coal particles commences at a short distance from the diffuser 10 and the flame front may be at an annular surface or region roughly in the position indicated by the broken wavy line 13.

The diffuser 10 is forwardmost or inner end of a tubular member 14 supported within which is the barrel 21 of the oil burner, which is pressure-atomizing and which compries an atomizer head 22 secured appropriately to the inner end of the barrel 21. At the outer end of the tubular member 14 the burner barrel 21 is slidably supported by part of a support bracket 23, which is secured to the tubular member 14 and carries a pneumatic cylinder (not shown) for moving the burner barrel 21 and the atomizer head 22 longitudinally of the tubular member 14, whereby the atomizer head 22 may be projected inwardly towards the combustion chamber 4 to the operative position shown in which it is nearer the combustion chamber than the pulverized fuel burner diffuser 10. Similarly, the burner barrel 21 and the atomizer head 22 may be retracted outwardly away from the combustion chamber to an inoperative position within the tubular member 14. When the oil burner atomizer head 22 is in the operative position and the oil burner alone'is operated and the oil flame therefrom is healthy or stable, the flame front may be at a line or zone roughly in the position indicated by the wavy chain line 24.

' The sighting pipe 6 lies vertically below the pipe 7, its inner end 25 is open and at a location below and somewhat outwardly of the diffuser 10. Its outer end 26 carries a housing 27 for a photoconductive cell or light sensing tube, and the sighting pipe is inclined so that radiation from the flame fronts can pass outwardlyv along it. Usually there will be a sight glass (not shown) at the outer extremity of the sighting pipe 6 and another sight glass (not shown) in a sighting tube 28 of the cell housing 27.

Referring to FIGURE 2, the photo conductive'cell41, arranged within the housing 27 to receive radiation travelling along the sighting pipe 6 (see FIG. -1), is preferably of Mullard type ORP.1'-2 and is connected in an electronic circuit in which it is energized by current which flows from a positive lead 42 in series to a negative lead 48 through a resistance 43 of 47 ohms, a first silicon junction diode 44 of Mullard type OAlZlO, the cell 41, a choke or inductance coil 45 of -1 henry and 80 ohms, a first transistor 46 (of p-np kind) from emitter to collector thereof, anda second silicon junction diode 47 of the same type as the first diode 44.

The transistor 46 is stabilized by the connection of the base thereof to the junction between a zener diode 49, i.e., a diode having a constant voltage across it, and a second resistance 50 of kilohms, which are connected for the flow of current from the positive lead 42 to the negative lead 48 in series through the zener diode 49 from cathode to anode thereof and through the said second resistance 50. A first condenser 51 of 100 microfarad capacity is connected between the junction of the cell 41 and the in ductance 45 and the junction of the zener diode 49 and the second resistance 50. j

The junction between the cell 41 and the first diode 44 is connected via a second condenser 52 of 1 microfarad capacity to the base of a second transistor 54 (of p-n-p kind). The base of second transistor 54 is connected through a third resistance 53 of 1 megohrn to the negative lead 48, and the emitter of second transistor 54 is connected to the positive lead 42, and the collector of second transistor 54 is connected through a fourth resistance 55 of 15 kilohms to the negative lead 48. The junction between the collector of the second transistor 54 and the fourth resistance 55 is connected through a third condenser 61 of 1 micr-ofarad capacity and in series therewith a fifth resistance 62 of 15 kilohms to the junction between the collector of the first transistor 46 and the second diode 47. The junction between the collector of the first transistor 46 and the second diode 47 is also connected through a fourth condenser 63 of -1 microfarad capacity and in series therewith a sixth resistance 64 of 22 kilohms to the base of a third transistor 65.

The third transistor 65 is of n-p-n kind and is energized by the connection of its collector through a seventh resistance 66 of 15 kilohms to the positive lead 42 and the connection of its emitter to the negative lead 48. The base thereof is connected through an eighth resistance 67 of l megohm to the positive lead 42. An output from the circuit described is taken from the collector of the third transistor 65 through a suitable condenser 68 of, for example, 0.47 microfarad capacity directly or through any suitable means to bistable trip circuits or any other suitable devices, indicated generally at 70, connected for the operation of such indicating or/ and alarm or/and control means as may be desired in the flame monitoring apparatus.

In operation of the flame monitoring apparatus, the photo-conductive cell 41 views the oil flame 'or/ and the pulverized coal flame at or near the flame front or fronts thereof receives therefrom light radiation of fluctuatingintensity. The electronic circuit shown in FIG. 2 responds according to the fluctuating illumination of the cell and provides a fluctuating output having alternating components therein which are representative of the high frequencies associated with the flame front. If the output has an undesirably high fraction therein of alternating components of considerably lower frequencies not normally associated with flame fronts, suitable means, for example a low frequency cutoff, might be arranged to assist the action of the condensers 52 and 61 in the fluctuating signal path in reducing or eliminating the fraction of low frequency components.

Fluctuations in the cell illumination cause fluctuations in the cell resistance and thus fluctuations in the cell ourrent. The choke 45 and the first condenser 51 co-operate to insure that the first transistor 46 passes substantially the average current, thev alternating components of the fluctuating current substantially by-pass the transistor 46 through the first condenser 51 and the second resistance 50. The fluctuating current passing in series through the first silicon diode 44 and the cell 41 causes a fluctuating signal to pass through the second condenser 52 to the second transistor 54. When this signal is amplified, it is transmitted through the third condenser 61 and fifth resistance 62 to the junction between the collector of the first transistor 46 and the second silicon diode 47 From this point a path' is available to the signal through the second silicon diode 47 and a second path through the fourth condenser 63 and the sixth resistance 64' to the third transistor 65 which provides the circuit output.

A healthy oil flame produces light at the flame front which has a ratio 'of a certain order of light radiation fluctuations of high frequency to average light radiations, and-the electronic circuit of FIG. 2 is adapted to produce an output according to the ratio of light fluctuations to average light in the illumination of the cell 41. Should, for instance, a sight glass in the radiation path from the flame to the cell 41 become dirty, the light radiation fluctuations at the cell 41 and the average light radiation at the cell are reduced in the same proportion. The reduction in the light radiation fluctuations at the cell reduces the amplitude of the fluctuating signal passed through the second condenser 52 to the second transistor 54. If it were not for the first diode 44, which has a characteristic that will be described, the reduction in the amplitude of said fluctuating signal would, by virtue of the phenomenon that at the reduced average radiation at the cell the cell responds less quickly to each separate fluctuation of light radiation, be considerably larger than corresponds to the reduction in light radiation fluctuations at the cell. However, the first diode 44 has a characteristic of alternating current resistance plotted against current which is practically hyperbolic and which therefore has a higher alternating resistance at the reduced cell current corresponding to the reduced average light radiation at the cell. Therefore, by virtue of the presence of this diode 44 the reduction in amplitude of the fluctuating signal is not larger than corresponds to the reduction in light radiation fluctuations at the cell; The diode 44 and first resistance 43 are effective to compensate for the less quick response of the cell at reduced illuminations. The first resistance 43 is required for the effect in the circuit described but may not be needed if a gold-bonded germanium diode is used instead of a silicon diode.

The second transistor 54 now passes the smaller but compensated fluctuating signal, the value of which could be indicated if information as to the flicker radiation at the cell weredesired, to the junction between the first transistor 46 and the second diode 47. Since the second diode 47 receives a smaller direct current, it has a larger A.C. resistance, its characteristic being the same as that ofthe first diode 44. Thus the path for the signal through the second diode 47 is of higher impedance and the value of the signal passed to the third transistor 65 is unchanged in spite of the partial obscuring of the radiation by the dirty sight glass.

If the healthy oil flame is replaced by a healthy pulverised coal flame, the illumination at the cell will be reduced, but the ratio of light radiation fluctuations to average light radiation will remain of the same order, and accordingly the circuit output signal will remain of the same order accordingly. If, however, an oil flame or a pulverized coal flame should become unhealthy, the light fluctuations will normally fall in relation to the average light and the circuit output will accordingly make a large drop which will be detected by the subsequent alarm and control apparatus. The arrangement described responds to flames of a Wide range of flame brightness and moreover involves no moving parts.

It will be understood that the output through the condenser 68 may be taken to a device that will indicate by display or recording, according to the value of the output, the value of the ratio of light fluctuations to average light in the illumination of the cell 41, whereby the supervisor may readily inform himself of the continuance or otherwise of stable ignition and combustion at the burner. Alternatively or in addition, the output may be taken to an alarm device that will operate an audible or/ and visual alarm upon the falling of said ratio below a predetermined value; alternatively or in addition, the output may be taken to a control device that will automatically, upon the falling of said. ratio below a predetermined value, effect control operations such as the cutting off of the fuel supply to the burner of which the flame is being monitored.

A further output from the circuit may be taken via connection 69 to any suitable device 71 for measuring the average or steady potential of the junction between the first diode 44 and the cell 41, thereby to produce for indicating purposes or/ and if desired for control purposes a signal representative of the average light radiation received at the cell. By reason of the connection, in series with the cell 41, of the first diode 44, of which the DC. resistance increases faster with current reduction the smaller the current, the said potential at the cell positive terminal varies at low illuminations to a greater extent with a given change in illumination than if merely a fixed resistance were in series with the cell. The output at 69, which may vary substantially according to a straight line law with the logarithm of the illumination of the cell measured in lumens per unit area, therefore constitutes a useful signal over a range extending to a lower level of illumination. The signal at 69, if the first output of the circuit through condenser 68 indicates healthy combustion, can, in conjunction with knowledge as to which burner is operating, act as a check on whether the sight glasses are sufliciently clean.

The circuit employs the second silicon diode 47 in a subcircuit or calculating network to take a ratio. The second diode 47 is fed with direct current from a high resistance source in the form of the first transistor 46 and simultaneously with an alternating voltage from a high impedance source in the form of the second transistor 54, and it can be shown that by virtue of the practically hyperbolic A.C. resistance-current characteristic of the diode, the alternating component of the voltage across the diode is substantially proportion-a1 to the ratio of A.C. to DC I claim:

1. Apparatus for monitoring a flame which exhibits a fluctuating illumination characteristic, said apparatus comprising an electronic circuit adapted to provide a signal dependent upon flame radiation fluctuations, said circuit including a photo-conductive cell disposed in light receiving relationship to said flame and having a characteristic whereby receipt of reduced average radiation from said flame effects a proportionately reduced cell current and a diminished response to said radiation fluctuations, and means for compensating for the diminished flame fluctuation response of said cell comprising a diode connected in series with said cell and having a characteristic A.C. resistance/current relationship whereby resistance is increased at said reduced cell current.

2. Apparatus for monitoring a flame comprising an electronic circuit adapted to provide a signal representative of the average flame radiation, said circuit including a photo-conductive cell disposed in light receiving relationship to said flame and having a characteristic whereby receipt of reduced average radiation from said flame effects a proportionately reduced cell current, and means for increasing the sensitivity of said signal to changes in the amount of said flame radiation received in said cell comprising a diode connected in series with said cell and having a characteristic D.C. resistance/current relationship whereby resistance is increased at said reduced cell current.

3. Apparatus for monitoring a flame which exhibits a fluctuating illumination characteristic, said apparatus comprising an electronic circuit having a photo-conductive cell disposed in light receiving relationship to said flame, and a calculating network including a diode having a substantially hyperbolic A.C. resistance/current characteristic, means including a first transistor for feeding a direct current representative of the average illumination received in said cell to said diode, and means including a second transistor for simultaneously feeding an alternating voltage representative of the fluctuations of said flame to said diode, whereby an output signal substantially proportional to the ratio of said direct current and said alternating voltage and representative of the ratio of flame fluctuations and average flame illumination is produced.

4. Apparatus for monitoring a flame which exhibits a fluctuating illumination characteristic, said apparatus comprising an electronic circuit including a photo-conductive cell disposed in light receiving relationship to said flame to produce a fluctuating current, said cell having a characteristic whereby receipt of reduced average radiation from said flame effects a proportionately reduced average cell current and a diminished response to said radiation fluctuations, a first diode connected in series with said cell and having a characteristic resistance/current relationship whereby resistance is increased at said reduced cell current to compensate for the diminished radiation fluctuation response of said cell, a second diode having a resistance/current relationship substantially similar to that of said first diode, means for passing said average cell current to said second diode, and means for passing an A.C. signal representative of radiation fluctuations on said cell to said second diode, whereby a fluctuating A.C. output signal representative of the ratio of flame radiation fluctuations to average flame radiation is produced.

5. Apparatus for monitoring a flame which exhibits a fluctuating illumination characteristic, said apparatus comprising an electronic circuit including a photo-conductive cell disposed in light receiving relationship to said flame to produce a fluctuating current, said cell having a characteristic whereby receipt of reduced average radiation from said flame effects a proportionately reduced average cell current and a diminished response to said radiation fluctuations, a first diode connected in series with said cell and having a characteristic resistance/current relationship whereby resistance is increased at said reduced cell current to compensate for the diminished radiation fluctuation response of said cell, a second diode having a resistance/current relationship substantially similar to that of said first diode, a transistor, means for passing said average cell current through said transistor to said second diode, and means for passing an AC. signal representative of radiation fluctuations on cell to said second diode, whereby a fluctuating A.C. output signal representative of the ratio of flame radiation fluctuations to average flame radiation is produced at the junction between said transistor and said second diode.

6. Apparatus for monitoring a flame which exhibits a fluctuating illumination characteristic, said apparatus comprising an electronic circuit including a photo-conductive cell disposed in light receiving relationship to said flame to produce a fluctuating current having alternating components, said cell having a characteristic whereby receipt of reduced average radiation from said flame effects a proportionately reduced average cell current and a diminished response to said radiation fluctuations, a first diode connected in series with said cell and having a characteristic resistance/current relationship whereby resistance is increased at said reduced cell current to compensate for the diminished radiation fluctuating response of said cell, a second diode having a resistance/current re- 7 lationship substantially similar to that of said first diode, means for passing an A.C. signal representative of radiation fluctuations on said cell to said second diode, a transistor, means for by-passing said alternating components around said transistor and said second diode comprising a choke in series with said cell, and a condenser connected between said cell and said choke and in parallel with said choke, and means for passing said average cell current through said transistor to said second diode, whereby a fluctuating A.C. output signal representative of the ratio of flame radiation fluctuations to average flame radiation is produced.

7. Apparatus for monitoring a flame which exhibits a fluctuating illumination characteristic, said apparatus comprising an electronic circuit including a photo-conductive cell disposed in light receiving relationship to said flame top produce a fluctuating current, said cell having a characteristic whereby receipt of reduced average radiation from said flame effects a proportionately reduced average cell current and a diminished response to said radiation fluctuations, a first diode connected in series with said cell and having a characteristic resistance/current relationship whereby resistance is increased at said reduced cell current to compensate for the diminished radiation fluctuation response of said cell, a Second diode having a resistance/current relationship substantiall similar to that of said first diode, a first transistor, means for passing said average cell current through said first transistor to said second diode, a second transistor, and means for passing an A.C. signal representative of radiation fluctuations on said cell to said second diode through said second transistor to amplify said A.C. signal, Whereby a fluctuating A.C. output signal representative of the ratio of flame radiation fluctuations to average flame radiation is produced.

8. Apparatus according to claim 7 wherein the resistance/current relationships of said first and second diodes are substantially hyperbolic.

References Cited by the Examiner UNITED STATES PATENTS WALTER STOLWEI N, Primary Examiner. 

6. APPARATUS FOR MONITORING A FLAME WHICH EXHIBITS A FLUCTUATING ILLUMINATION CHARACTERISTIC, SAID APPARATUS COMPRISING AN ELECTRONIC CIRCUIT INCLUDING A PHOTO-CONDUCTIVE CELL DISPOSED IN LIGHT RECEIVING RELATIONSHIP TO SAID FLAME TO PRODUCE A FLUCTUATING CURRENT HAVING ALTERNATING COMPONENTS, SAID CELL HAVING A CHARACTERISTIC WHEREBY RECEIPT OF REDUCED AVERAGE RADIATION FROM SAID FLAME EFFECTS A PROPORTIONATELY REDUCED AVERAGE CELL CURRENT AND A DIMINISHED RESPONSE TO SAID RADIATION FLUCTUATIONS, A FIRST DIODE CONNECTED IN SERIES WITH SAID CELL AND HAVING A CHARACTERISTIC RESISTANCE/CURRENT RELATIONSHIP WHEREBY RESISTANCE IS INCREASED AT SAID REDUCED CELL CURRENT TO COMPENSATE FOR THE DIMINISHED RADIATION FLUCTUATING RESPONSE OF SAID CELL, A SECOND DIODE HAVING A RESISTANCE/CURRENT RELATIONSHIP SUBSTANTIALLY SIMILAR TO THAT OF SAID FIRST DIODE, MEANS FOR PASSING AN A.C. SIGNAL REPRESENTATIVE OF RADIATION FLUCTUATIONS ON SAID CELL TO SAID SECOND DIODE, A TRANSISTOR, MEANS FOR BY-PASSING SAID ALTERNATING COMPONENTS AROUND SAID TRANSISTOR AND SAID SECOND DIODE COMPRISING A CHOKE IN SERIES WITH SAID CELL, AND A CONDENSER CONNECTED BETWEEN SAID CELL AND SAID CHOKE AND IN PARALLEL WITH SAID CHOKE, AND MEANS FOR PASSING SAID AVERAGE CELL CURRENT THROUGH SAID TRANSISTOR TO SAID SECOND DIODE, WHEREBY A FLUCTUATING A.C. OUTPUT SIGNAL REPRESENTATIVE OF THE RATIO OF FLAME RADIATION FLUCTUATIONS TO AVERAGE FLAME RADIATION IS PRODUCED. 